Because the use of traditional lead (Pb) shot has been outlawed for waterfowl hunting in the U.S., Canada, U.K. and other countries, much effort has been devoted to identifying a suitable substitute. To be fully satisfactory, alternative shot must possess the following attributes:
a) The material should have density similar to that of lead (Pb) shot, which is typically 11.0 g/cc. For example, it should have a density of at least approximately 8.5 g/cc.
b) The material should not cause physiological problems in wildlife which may ingest spent shot from the ground or water. Accordingly, it should be non-toxic.
c) The material should not cause significant damage to shotgun barrels. Preferably, it is softer than conventional gun barrel steel or utilized in a cartridge adapted to protect the gun barrel.
d) The projectiles should possess sufficient strength, rigidity and toughness to adequately withstand “set-back” forces associated with firing.
In addition to the above, effective lead substitutes may (but are not required to) be magnetic to easily differentiate it from illegal lead projectiles. When possible, another non-required goal is to make the material used for firearm projectiles economical to obtain and fabricate into firearm projectiles.
When selecting a material to be used instead of lead for firearm projectiles, factors that may be considered include the cost of the material, the expense of processing the material, the toxicity of the material, the density of the resulting projectile, and the strength of the resulting projectile. Examples of materials that have been used with various degrees of success to replace lead, and their approximate densities, include steel (7.9 g/cc), bismuth (9.4 g/cc), zinc (7.14 g/cc), tin (7.3 g/cc), and tungsten (19.3 g/cc). Tungsten and bismuth tend to be much more expensive the lead, although the increased expense of tungsten and its alloys needs to be balanced with the significant density of these materials. Examples of effective tungsten-based lead substitutes are disclosed in U.S. Pat. Nos. 6,447,715, 6,270,549, and 6,527,880, the complete disclosures of which are hereby incorporated by reference.
A consideration when using a lower density material, such as iron or other materials with a density of approximately 8 g/cc or less, is that projectiles formed from these materials will carry less energy at a given velocity and experience a more rapid loss of velocity (such as due to drag forces) than a lead or other higher density object having the same size and shape. In the context of firearm projectiles, lower density projectiles will tend to have less stopping power, flight trajectory and/or range than conventional lead or lead-antimony projectiles.
Steel Shot
The most widely used alternative shot is carbon steel, in spite of the fact that its density is quite low (about 7.9 g/cc) in comparison with that of lead shot (about 11.0 g/cc). Inarguable principles of physics and engineering establish that an object of lower density, when moving through a fluid (such as air), will carry less energy at any given velocity, and experience more rapid loss of velocity (due to drag forces) than an object of higher density of the same size and shape. Shotshell manufacturers have employed special powders to increase steel shot velocity, in an attempt to ameliorate its inferior ballistic properties. The “hotter” powders unfortunately create higher pressures within the gun barrel. Safety considerations have therefore prompted shotshell manufacturers to recommend that steel shells only be fired in certain types of modern, high-strength shotguns.
Bismuth Shot (U.S. Pat. No. 4,949,644 to Brown)
Bismuth alloy shot (approximately 9.4 g/cc is somewhat more dense than steel (7.9 g/cc) but not as dense as lead-antimony alloys (approximately 11-11.3 g/cc) that are conventionally used for shot and other projectiles. However, bismuth shot is considerably more expensive than lead, steel and similar shot. In addition to this shortcoming, bismuth alloys are inherently brittle and therefore tend to fracture and disintegrate upon impact (January, 1998 issue of Gun Tests). As fracture surfaces form in the shot, energy is lost which would otherwise be available to enhance penetration of the target. In this instance, it is even likely that all the increased energy gained by having higher density than steel is lost as fracture occurs. Finally, it should be noted that bismuth is non-magnetic and cannot be readily distinguished from illegal lead shot by game officers in the field.
Iron-Tungsten Shot (U.S. Pat. Nos. 5,264,022, 5,527,376 and 5,713,981 Assigned to Teledyne Industries, Inc.)
A product which began to be marketed in the USA in 1997 is a shotshell containing binary iron-tungsten alloy shot (60% Fe-40% W, by weight). Because the Fe—W is very hard (about Rockwell C50), and therefore must be ground with ceramic abrasives (alumina, silicon-carbide, diamond, etc.), particles of which become imbedded in the shot surface, this type of shot will result in severe damage in all gun barrels unless the shot is encapsulated in a special “overlapping double-wall” plastic shot cup of heavy construction. However, a theory is that it is possible for a few shot to rebound forward out of the plastic cylinder upon firing and to thereby contact the unprotected steel barrel. The consequences of forming longitudinal scratches on the barrel are that stresses produced by the expanding explosive gases will be concentrated in the regions around the scratches. A primary concern is that these stresses may be sufficiently high to cause bursting of the barrel.
Whether adequately protective or not, the special plastic shot cup (or “Wad”) renders it impossible to load quantities of shot equivalent to those of traditional lead shells. For example, Fe—W shells of 2¾-inch length for 12-gauge guns contain only 1.0 ounce of shot versus 1⅛ to 1¼ ounces in corresponding lead or steel shells. The deficient pellet numbers result in correspondingly sparse pattern densities, the same problem encountered in substituting larger steel shot for traditional lead sizes, as mentioned previously. This should not preclude the use of these shot cups, or lead substitutes that require the use of these shot cups, but it is a consideration, or trade off, to be evaluated.
Finally, problems associated with manufacturability, and their adverse effects on product cost, are relatively severe. The constituent phases in Fe—W alloys cause the shot to be so hard and brittle as to be impossible to forge or swage these alloys into rods, or even to shape them compressively into spheres. Although the referenced patents claim Fe—W shot can be made by casting, the inherent brittleness and high melting temperatures of these alloys caused cracking to occur during rapid cooling. Cracking also plagued the process of compressive grinding, which was tried as a means of rounding the generally asymmetrical shot. Consequently, the shot actually being produced and marketed must be made by an expensive powder metallurgical method. Even with this approach, it is difficult to make cost effective smaller shot sizes with this method. This is due to the fact that powder processing costs increase exponentially as shot sizes decrease. Furthermore, the fragility of compaction tooling becomes a limiting factor as shot size decreases. Shot sizes #4 (0.130-inch), #5 (0.120-inch), #6 (0.110-inch) and #7½ (0.095-inch), traditionally preferred for hunting all but the very largest game birds (such as geese), are unavailable for these reasons.
Tungsten-Polymer Shot
A newer version of an older idea (U.S. Pat. No. 4,949,645 to Hayward et al.) is currently proposed for the U.S. market in 1998-1999 (January/February, 1995 issue of Ducks Unlimited Magazine and March, 1998 issue of Petersen's Shotguns). This shot material is a composite of tungsten powder and a powdered thermoplastic polymer. Mixtures of these two constituents are formed into spheres of cured composite, the polymer “glue” being the continuous phase and the tungsten powder particles the discontinuous phase. By virtue of its weak polymer-to-metal bonds, the material will reportedly not damage gun barrels. It is this very “weakness,” however, which is one of the undesirable features of tungsten-polymer shot. Rigidity and strength are important material properties which affect the ability of shot to (1) penetrate the target effectively, and (2) remain spherical during launching and flight.
The penetrability factor can be easily understood by considering the behavior of a rubber bullet (used, for example, by police). The projectile does not penetrate well because its kinetic energy is absorbed and dissipated by its own deformation. Rigidity, as used here, is measured by a material property value known as elastic modulus. Because the elastic moduli of all organic polymers are far lower than those of metals, the subject composite materials are, as expected, less rigid than steel, Fe—W, et al. The second factor is important when a different type of shot distortion/deformation occurs which causes loss of sphericity, thereby degrading shot pattern density and uniformity. During firing, the shot experiences high compressive “set-back” forces. Materials which are relatively weak (i.e., low in yield strength), undergo various degrees of permanent distortion, referred to as “plastic deformation.” Any loss of sphericity will result in erratic flight paths of shot and will therefore produce undesirable pattern uniformity.
Another disadvantage of tungsten-polymer shot is one of economics. Because polymers are much lower in density than common metals such as iron, a composite density equivalent to that of lead-antimony shot (11.0 g/cc) can only be attained by using high concentrations (e.g., 95%) of costly tungsten powder. This disadvantage is reduced, to some degree, when it is desired to produce shot or other projectiles having a density that is less than that of lead-antimony shot; however, even a relatively small amount of polymer, such as more than a few wt % will substantially increase the amount of expensive tungsten, or tungsten-containing, powder needed to produce even this lower density shot or other projectile.
Other Prior Art
Other proposed shot materials include significant concentrations of lead as a specified ingredient. However, rulings by the U.S. Fish and Wildlife Service have outlawed the use of any shot material containing more than 1.0% lead. This action has eliminated consideration of proposed materials described in a variety of U.S. Patents: U.S. Pat. No. 2,995,090 to Daubenspeck; U.S. Pat. No. 3,123,003 to Lange, Jr. et al.; U.S. Pat. No. 4,027,594 to Olin; U.S. Pat. No. 4,428,295 to Urs; U.S. Pat. No. 4,881,465 to Hooper; and U.S. Pat. No. 5,088,415 to Huffman et al. are examples.
Even materials which are lower in density than steel have been proposed for alternative shot. Examples are zinc (7.14 g/cc) and tin (7.3 g/cc), the latter being reported in the Sep. 4, 1997 issue of American Metals Market. Such materials certainly offer no improvement in ballistic properties over those of steel shot.
Objects and Advantages
The present disclosure addresses the problems associated with other alternative materials for forming firearm projectiles. Several objectives of the present disclosure, which may be achieved individually or in groups according to various aspects of the present disclosure, include:
a) to provide a material which is castable and formable and therefore able to be manufactured by conventional processes;
b) to provide a material which will produce a firearm projectile having a density of at least 8 g/cc and preferably at least 8.5 g/cc;
c) to provide a material which is fully as dense as conventional lead-antimony alloys (11.0 g/cc) or higher;
d) to provide a material which has a density and performance characteristics that exceed those available from steel shot or other projectiles;
e) to provide a material which, unlike Fe—W and high-carbon steel, is softer than gun barrel steels, thereby reducing or eliminating damage;
f) to provide a material which is non-toxic to wildlife and the environment;
g) to provide a material which, if desired, can be made magnetic for game-law purposes;
h) to provide a material which will not fracture or disintegrate upon impact;
i) to provide a material which will produce frangible projectiles;
j) to provide a material which is strong enough to withstand firing without distorting (but soft enough to minimize gun barrel damage);
k) to provide a material which, by virtue of its softness, is suitable for use with conventional plastic wads used for low-carbon steel, thereby making it possible for private parties to load and use it;
l) to provide a material which, by virtue of its ferromagnetic properties, may be readily salvaged for reuse;
m) to provide a castable material having a density in the range of 8-10.5 g/cc; and
n) to provide a castable material having a density in the range of 10.5-15 g/cc.